Multi-target scrubber

ABSTRACT

A gas scrubber is presented. The gas scrubber comprises a nozzle that atomizes a liquid to form droplets. The droplets are preferably expelled from the nozzle in substantially a hollow cone spray pattern with a velocity of at least 4000 feet per minute. A stream of gas containing particulates that requires scrubbing interacts with the droplets. After the interaction, the gas-droplet combination impinges on a target. Preferred targets include droplets from a second nozzle, a ducting surface, or a throated passage.

This application claims priority to provisional application having U.S.Ser. No. 60/899,766 filed Feb. 5, 2007. This and all other extraneousmaterials discussed herein are incorporated by reference in theirentirety. Where a definition or use of a term in a reference, which isincorporated by reference herein, is inconsistent or contrary to thedefinition of that term provided herein, the definition of that termprovided herein applies and the definition of that term in the referencedoes not apply.

FIELD OF THE INVENTION

The field of the invention is liquid scrubbers for flue gasses.

BACKGROUND

There are numerous gas scrubbers in existence, including for examplescrubbers that operate upon flue gasses of power plants. When liquidabsorbents are used, the liquid is sprayed into the gas stream either ina counter-current or a cross-current configuration. Problems arisebecause the gas tends to flow in a laminar fashion around the fluiddroplets, which reduces effectiveness of the scrubbing.

Basic principles of scrubbing are set forth in Schiffner, Kenneth C., etal., “Wet Scrubbers”, 2^(nd) Ed, Technomic Publishing Co., Inc., pp1-10; Buonicore, Anthony J., et al., “Air Pollution Engineering Manual”,Air and Waste Management Ass'n, pp 78-88; Cooper, David C. et al., “AirPollution Control”, 3^(rd) Ed, Waveland Press, pp 115-118, 209-238.

One solution is to direct the sprays against a target barrier throughwhich the gas is flowing. Exemplary targeted barriers include plasticballs, and metal or ceramic saddle rings. Disruptions of the spray andgas streams caused by the barrier facilitate interaction of the sprayand gas, but considerable energy is expended to force the gas throughthe barrier at sufficient velocity to provide adequate scrubbing.

Patent publication WO 84/03641 to Jones describes an improved rotaryscrubber that uses a rotating mechanical atomizer. The rotating atomizerdisperses high velocity water droplets in a radial direction that iscross-current to the gas stream flow. However, the water droplets canrob the gas of forward momentum reducing the ability for interactionwith downstream targets.

Thus, there is still a need for apparatus and methods that facilitatescrubbing of gasses and liquid absorbents in a scrubber.

SUMMARY OF THE INVENTION

The present invention provides apparatus and methods in which a gascomprising particulates interacts with liquid droplets expelled from anozzle.

In a preferred embodiment, a nozzle forms the droplets by expelling theliquid in a cone spray pattern, preferably a hollow cone, at an averagevelocity of at least 4000 feet per minute (FPM). A stream of gascontaining particulates that require scrubbing interacts with thedroplets. The combined mixture then impinges on a target.

All suitable nozzles are contemplated. However, nozzles that provide ahollow cone spray pattern are more preferred over solid cone spraypatterns for some applications. Nozzles that provide a hollow cone spraypattern spay the liquid into a ring-shaped impact area where at least90% of the liquid falls within the ring area. The droplets fromneighboring nozzles act as a target for the stream of gas.

Other targets are also contemplated including a ducting surface, apack-tower, or a target having a throated passageway.

Various objects, features, aspects and advantages of the inventivesubject matter will become more apparent from the following detaileddescription of preferred embodiments, along with the accompanyingdrawings in which like numerals represent like components.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic of an exemplary test scrubber stage where a targetis droplets from a second nozzle.

FIG. 2 is a schematic of an exemplary test scrubber stage where a targetis pack-tower having a fill material.

FIG. 3 is a schematic of an exemplary test scrubber stage where a targetis a ducting surface.

FIG. 4 is a schematic of an exemplary test scrubber stage where a targethas a throated passageway

FIG. 5 is a schematic of an example four stage multi-stage scrubberhaving nozzles, pack-tower targets, and targets having throatedpassageways.

DETAILED DESCRIPTION

Preferred scrubber systems typically comprise multiple target stagesstrategically placed in series and that include spray nozzles andmultiple target barrier objects. A scrubbing liquid is sprayed atelevated pressure in target stages to provide the required relativeimpact energy necessary impinge the fine particulate matter or reactwith acid gases within a particulate-containing gas that requiresscrubbing. Example particulate-containing gases include flue gasses froma power plant containing NOx, SOx, particulate-matter, heavy metals,aerosols, odors, acids, or other pollutants can be scrubbed using thedisclosed techniques.

In FIG. 1, scrubber stage 100 comprises at least one of nozzle 110 thatexpels a liquid into spray 130. As gas stream 120 enters scrubber stage100, stream 120 interacts with a plurality of droplets within spray 130.The droplets carry the gas to a target comprising a plurality ofdroplets from spray originating from other nozzles. One should note thatscrubber stage 100 can represent a single stage of a larger scrubbersystem where each stage can include one or more of stream 120, one ormore of nozzle 110, or one or more scrubbing targets.

In a preferred embodiment, stream 120 comprises a particulate-containinggas that requires scrubbing and is oriented to interact with a pluralityof droplets within spray 130. Stream 130 is oriented to have a velocitythat is moderately parallel to, and in the same direction as the flow ofspray 130. Stream 120 carries the gas in a direction that is no greaterthan 30 degrees off of a centerline of spray 130. More preferably,stream 120 carries the gas in a direction that is no greater than 10degrees off of the centerline of spray 130.

The magnitude of the gas's velocity is preferably in the range fromabout 750 FPM to 1500 FPM, and more preferably no greater than 1000 FPM.However, it should be noted that it is contemplated that higher gasvelocities (e.g. up to 3000 FPM) can also be used to for applicationsincluding scrubbing of SO₂, HCl, or NH₄. As the gas in stream 120interacts with droplets in spray 130, the droplets carry the gasdownstream to impinge on one or more targets.

Configuring scrubber stage 100 to have stream 120 and spray 130 flowingin the same general direction allows the combined gas-droplet mixture toimpinge on other targets or other scrubber stages downstream with highvelocity. Impinging a target with high velocity ensures that gas anddroplets mix more efficiently due to further atomization or turbulentflow caused by the target.

Spray 130 preferably comprises a substantially hollow cone spray patternwhere droplets of liquid are concentrated into ring-shape as the liquidis expelled from nozzle 110. A hollow cone spray pattern provides for anefficient use of absorbent liquid by ensuring that most of the expelledliquid is concentrated near the outer surface of the cone where stream120 can interact with droplets easily. If the pattern were a solid cone,then a portion of the liquid would be shielded from stream 120 resultingin an inefficient use of liquid. One should note that other spraypatterns or their combinations are also contemplated including solidcones, flat spray, solid streams, or other patterns. For example, solidcone spray patterns provide acceptable performance when impacting atarget media or packing.

All nozzles are contemplated, although a preferred nozzle 110 isconfigured to expel droplets of a liquid in a hollow cone spray patternat high velocity. Suitable nozzles include Quick WhirlJet®, SpiralJet®or UniJet® hollow cone spray nozzles available from Spraying SystemsCo.® (http://www.spray.com). The WhirlJet product line comprises a whirljet nozzle providing acceptable atomization at low pressures and aneffective airborne impingement. The SpiralJet product line comprises aspiral shaped nozzle having a precision impact blade angle thatdistributes droplets efficiently over well defined ring-shaped coveragearea.

Nozzle 110 atomizes an absorbent liquid to form droplets in spray 130.Nozzle 110 is preferably configured to form droplets having averagediameter in the range from about 100 to about 300 micron as measuredusing a Volume Medium Diameter (VMD). When a range is specified herein,the range is consider to be inclusive the range's endpoints. A VMDrepresents the droplet size where 50% of the liquid volume of liquid isin the form of droplets having diameters less than the VMD and 50% ofthe volume liquid volume is in the form of droplets having a diametergreater than the VMD.

Smaller droplets are preferred over larger droplets to increase thesurface area to volume ratio of the droplets. Providing a maximalsurface area per unit volume of liquid ensures that stream 120 interactswith spray 130 efficiently. However, one should note that if thedroplets are too small (e.g. less than 100 microns), spray 130 willbloom and the liquid will quickly loose momentum. Conversely, if thedroplets are too large (e.g. greater than 300 microns), spray 130 willdrop too quickly reducing the efficiency.

In preferred embodiments, for example, the liquid is pressurized tobetween 30 PSI and 120 PSI, compared with less than 20 PSI inconventional systems. More preferred embodiments pressurize the liquidto between 60 PSI and 90 PSI, with still more preferred embodimentspressurizing the liquid to about 80 PSI.

Nozzles 110 can be arranged within scrubber stage 100 into nearly anyconfiguration. A preferred configuration includes placing a plurality ofnozzles 110 into an array where spray 130 from a first nozzle 110overlaps spray 130 of a second nozzle 110. The array is configured withoverlapping sprays 130 to ensure that stream 120 passes through spray130 without passing through a gap. Although a preferred embodiment hasan array of uniform nozzles 110, it is also contemplated that the arraycould include dissimilar nozzles.

One should note that overlapping sprays 130 impact each other with greatvelocity. As sprays 130 impinge on one another, a turbulent, boilingimpact zone is formed providing a target reaction area for scrubbingstream 120. Each of spray 130 can become a target for gas in stream 120carried by droplets from another spray.

Table 1 provides example nozzle characteristics for an array design toscrub a gas stream having a duct flow rate of 500 to 1000 FPM.

TABLE 1 Nozzle Characteristic Value Nozzle Spray Pattern Hollow ConeNozzle Type Spiral or Whirl Spray Angle 60 to 120 Deg Flow Rate 10 to 50gpm Pressure 40 to 120 PSI Material 316, 44OC, Plastic, Alloy

Gas stream 120 has a relatively low flow rate (e.g. less than 1000 FPM)to provide a desired level of scrubbing. The rate at which droplets areexpelled in spray 130 is preferably much greater (e.g. by a factor of atleast two) than the flow rate of stream 120 to properly impinge fineparticulate matter and to produce high velocity spray rebound in theoverlapping impact zone. Higher differential velocities provides for ahigher capture efficiency of particulate matter (e.g. PM10, PM2.5, oraerosols including those under 0.5 microns). These impact zones areformed as the concurrent sprays impact other sprays, packing, fill,barrier media (e.g. chevron, angled, or rectangular shaped impactmedia), or other target material.

A low gas velocity also provides for additional time related benefits.For example, a low gas flow rate allows for coalescence or adhesion ofparticle-to-particle or particle-to-droplet by mutual attraction,Brownian motion, or other random movement. Low gas velocity also ensuresthat the pressure drop from stage to stage is minimized.

Preferably, nozzle 110 expels droplets with a high velocity having anaverage velocity greater than 4000 FPM and more preferably having anaverage velocity greater than 8000 FPM. A high velocity spray carriesthe gas in stream 120 downstream to one or more downstream targets wherefurther scrubbing can be achieved.

Spray 130 preferably includes absorbent liquid comprising water.Preferred liquids are at least 80% water by weight where the remainingportion of the liquid can includes other materials useful for scrubbingstream 120. For example, the remaining 20% of the liquid in spray 130can include various reagents or other compounds. Contemplated compoundsinclude particulates or heavy metal salts, surfactants, chargedsurfactants (e.g. anionic or cationic), liquids with hydrostatic charge,calcium carbonate for scrubbing SO₂, hydroxides, or other knownscrubbing compounds.

It is also contemplated that additional scrubbing materials includedissolved or suspended solids entrained in the liquid. The solids caninclude reactive or inert solids as determined by the objectives of thescrubbing stage. When solids are entrained in the liquid, the solidspreferably comprise at least 10% of the liquid by weight. Example solidsinclude buffering agents, reagents containing magnesium, salts, or otherknown scrubbing solids.

In FIG. 2, scrubber stage 200 includes pack-tower target 240 having afill material 250 used to enhance further scrubbing of stream 120 byinteracting with spray 130. It is contemplated that scrubber stage 200can be used as one or more stages within a larger scrubbing system.

Tower 240 preferably comprises a pack-tower. A pack-tower can be anysuitable dimensions and can include filler material 250 packed in thetower. Preferably fill material 250 has a thickness of 8 inches to about12 inches. Fill material 250 comprises a loose fill randomly disposedwithin tower 240 and allows stream 120 and spray 130 to enter tower 240.Example file material includes target balls preferably have a diameterof about 3.5″, saddle rings having dimensions of about 1¾″ long, by ¾″wide, by 1¼″ high, mist eliminator chevrons, woven wire products, orother high-surface area, non-plugging media.

A typical penetration depth of spray 130 within tower 240 is about 12inches with a preferred range of about 8 inches to about 18 inches.Deeper penetration is also contemplated. However, spray 130 looses muchof its penetration energy at higher depths.

In FIG. 3, scrubber stage 300 includes one or more ducting targets 340.Gas stream 120 interacts with droplets expelled by nozzles 110 withinspray 130. After the interaction, the combined gas-droplet mixture isfunneled together via the ducting surfaces of ducting targets 340.

Ducting targets 340 preferably comprise one or more surfaces acting asbaffles to further enhance scrubbing by forcing additional interactionbetween stream 120 and spray 130. Ducting targets 340 can comprise anysuitable shape or orientation that provides for further interaction.

In FIG. 4, scrubber stage 400 includes throated targets 440 wherethroated targets 440 have a narrow passageway and form annular targetswith spray 130. An annular target comprises a spray impact area on asection of material (e.g. metal, plastic, or other material) shaped tohave a converging surface configured in approximately a cone-shape.

In a preferred embodiment having throated targets 440, each of nozzles110 has a corresponding throated target 440. Throated target 440 issized and dimensioned to have a narrowed throat that is smaller than thering-shaped impact area of a hollow cone spray. Spray 130 impacts theentrance to the throat forming a curtain through which stream 120 mustpass. The throat also forces addition interaction between droplets fromspray 130 and stream 120 as they pass through the narrow passageway.

Throated targets 440 preferably have a converging conical surface, athroat, and a rear diffuser. The surface of the conical opening ofthroated targets 440 can be of any shape including a true cone, apyramidal structure, a hexagonal structure, an octagonal structure, orother shape that forces the convergence of droplets from spray 130 andgas stream 120. Spray 130 impacts the conical surface forming an annulartarget due to the impact area of the spray ring. Spray 130 rebounds andcontinues with gas stream 120 through the narrowed throat to the reardiffuser which aids in reducing pressure drop within the scrubber duct.A preferred throated passageway is sized and dimensioned to eliminatepressure losses of greater than one inch of water within the scrubberducts.

In FIG. 5 example four-stage scrubber 500 incorporates the elementsdisclosed above to scrub gas stream 520 of pollutants including NOx,SOx, odor, or other unwanted material. Scrubber system 500 comprisesfour stages, each including a pack-tower target 540. Nozzles 510 expelliquids to interact with stream 520. In the first and third stage,nozzles 510 spray liquid into one or more of throated targets 530.

Table 2 includes example nozzle characteristics for scrubber stages thathave an annular target or a pack-tower scrubber having packing or otherfill material. It should be noted that the characteristics in Table 2are examples and that the characteristics (e.g. angle, flow, pressure,etc. . . . ) can be varied as desired to fit the requirements of thescrubber.

TABLE 2 Nozzle Characteristic Multiple Annular Targets Packing, Fill,etc Capacity 3,000 CFM ea. Nominal 500 to 1000 FPM duct velocity NozzleSpray Pattern Hollow Cone Full Cone Nozzle Type Spiral, Whirl, otherSpiral, Whirl, other Spray Angle 50 to 90 Deg 60 to 120 Deg Flow 8 to 25GPM 10 to 50 GPM Pressure 80-120 PSI 40 to 120 PSI Material 316, 440′2,Plastic, Alloy 316, 44OC, Plastic, Alloy

It is contemplated that a multi-stage scrubber can incorporate one ormore stages where each stage includes desired scrubbing elements drawnfrom the disclosed subject matter. It should be noted that variation inscrubbing design characteristics from stage to stage also falls withinthe scope of the inventive subject matter. For example, the first stageof scrubber 500 could use water an absorbent liquid while the thirdstage could use a liquid containing a peroxide reagent. Additionally,nozzles 510 in the first stage could comprise hollow cone spray patternswhere the forth stage uses full or solid cone spray patterns.

It should be apparent to those skilled in the art that many moremodifications besides those already described are possible withoutdeparting from the inventive concepts herein. Moreover, in interpretingthe disclosure, all terms should be interpreted in the broadest possiblemanner consistent with the context. In particular, the terms “comprises”and “comprising” should be interpreted as referring to elements,components, or steps in a non-exclusive manner, indicating that thereferenced elements, components, or steps could be present, or utilized,or combined with other elements, components, or steps that are notexpressly referenced. Where the specification claims refers to at leastone of something selected from the group consisting of A, B, C . . . andN, the text should be interpreted as requiring only one element from thegroup, not A plus N, or B plus N, etc.

What is claimed is:
 1. A gas scrubber comprising: a nozzle that expelsdroplets of a liquid in a substantially hollow cone spray pattern at anaverage velocity of at least 4000 FPM; a stream of aparticulate-containing gas oriented parallel to and in a flow directionof the droplets such that the droplets carry the particulates in theflow direction; a target comprising at least one of (1) a plurality ofpacking elements and (2) a ducting surface; and wherein the target isdisposed such that the particulates impinge the target along with thedroplets to thereby increase interaction of the stream with the dropletsin an arrangement such that the interaction enhances scrubbing of thestream.
 2. The scrubber of claim 1, wherein the nozzle is a spiralshaped nozzle.
 3. The scrubber of claim 1, wherein the nozzle is a whirljet nozzle.
 4. The scrubber of claim 1, wherein the droplets areproduced having a volume medium diameter in the range from about 100microns to about 300 microns.
 5. The scrubber of claim 1, wherein thenozzle expels the droplets at an average velocity of at least 8000 FPM.6. The scrubber of claim 1, wherein the stream carries the gas at avelocity of no greater than 1000 FPM.
 7. The scrubber of claim 1,wherein the stream carries the gas in a direction that is no greaterthan 30 degrees off a centerline of the hollow cone spray pattern. 8.The scrubber of claim 7, wherein the stream carries the gas in adirection that is no greater than 10 degrees a centerline of the hollowcone spray pattern.
 9. The scrubber of claim 1, wherein the targetcomprises a plurality of droplets from a second nozzle.
 10. The scrubberof claim 9, wherein the second nozzle has a spray pattern that overlapsthe nozzle's hollow cone spray pattern.
 11. The scrubber of claim 1,wherein the target comprises a ducting surface.
 12. The scrubber ofclaim 1, wherein the target comprises pack-tower that has the pluralityof packing elements.
 13. The scrubber of claim 11, wherein the targetcomprises a converging throated passageway.
 14. The scrubber of claim13, wherein the throated passageway is sized and dimensioned toeliminate pressure losses of greater than one inch of water.
 15. Thescrubber of claim 1, wherein the liquid is at least 80% water.
 16. Thescrubber of claim 1, wherein the liquid comprises at least 10% by weightentrained solids.
 17. The scrubber of claim 1, wherein the targetcomprises a plurality of packing elements.